The present invention relates to a voltage detection device and an insulation detecting apparatus for a non-grounded power supply includiing the voltage detection device and, more particularly, to a voltage detection device for detecting a voltage using a microcomputer and an insulation detecting apparatus for a non-grounded power supply including the voltage detection device.
A voltage detection circuit for detecting a voltage using a microcomputer is equipped with a clamping diode for preventing the application of an excessive positive or negative voltage which can damage the microcomputer to an input port of the microcomputer to which a part subjected to voltage detection is connected. A rectifier diode may be used as the clamping diode to prevent the application of an excessive positive or negative voltage to the input port of the microcomputer for a relatively short time. However, in order to prevent the application of an excessive positive or negative voltage to the input port of the microcomputer for a relatively long time, a Schottky barrier diode, which has a smaller forward voltage drop, is used to reduce stress on the microcomputer.
However, since a Schottky barrier diode has a character that a leakage current at the same becomes greater as the ambient temperature increases, a voltage drop or voltage loss through the Schottky barrier diode increases with the ambient temperature. For this reason, a voltage detection circuit having a Schottky barrier diode has a problem in the accuracy of a detection voltage detected at an input port of a microcomputer.
Under the circumstance, one approach for the improvement of the accuracy of the detection voltage is to correct the detection voltage through detection of the ambient temperature which is achieved by connecting a thermistor to an input port of the microcomputer different from the input port connected to the part under voltage detection. In this case, in addition to connecting the thermistor to the input port of the microcomputer different from the input port connected to the part under voltage detection, data associated with a relationship between the ambient temperature and a leakage current at the Schottky barrier diode are input in advance to the microcomputer. The microcomputer calculates the ambient temperature from a voltage detected at the input port connected with the thermistor and converts the calculated ambient temperature into a leakage voltage using the data associated with a relationship between the ambient temperature and a leakage current. Further, the value of any voltage loss is identified based on the converted leakage current to correct the voltage detected at the input port of the microcomputer connected to the part under voltage detection using the value of the voltage loss thus identified.
In such a voltage detection circuit, the ambient temperature is calculated using the thermistor; conversion is performed to find any leakage current at the Schottky barrier diode based on a theoretical relationship between the ambient temperature and a leakage current; and a loss is identified based on the leakage current identified as a result of the conversion. However, the characteristics of a Schottky barrier diode do not necessarily agree with specifications shown on the data book and may include variations. Therefore, the calculation and conversion are repeated with a possibility of an error, and an error between a calculated value of a voltage loss and the actual value of the voltage loss can increase each time a calculation or conversion is repeated. For this reason, in such a voltage detection circuit using a thermistor, it is difficult to improve the accuracy of a detection voltage detected at the input terminal of the microcomputer.
As an apparatus having such a voltage detection circuit, the inventors conceived an insulation detector including: a first switching unit which connects a capacitor to a DC power supply whose wirings at a positive terminal side and a negative terminal side thereof are insulated from a part at ground potential, the capacitor being connected in series for a first preset time, a second switching unit which connects the capacitor between the positive terminal of the power supply and the part at ground potential, the capacitor being connected in series for a second preset time, a third switching unit which connects the capacitor between the part at ground potential and the negative terminal of the power supply, the capacitor being connected in series for the second preset time, a fourth switching unit which connects a voltage detection circuit for detecting a voltage across terminals of the capacitor after each of the first, second, and a third switching unit is unmade, and a calculation unit which estimates a power supply voltage of the power supply based on the voltage detected by the voltage detection circuit after the first switching unit is unmade and finding an insulation resistance of the power supply to the part at ground potential based on the estimated power supply voltage of the power supply and each of the voltages detected by the voltage detection circuit when the second and third switching unit are unmade.
In such an insulation detector, the first preset time is set shorter than the time required for fully charging the capacitor; the capacitor is charged for the first preset time by connecting it between the DC power supply and the part at ground potential in series with the first switching unit; and the voltage across the terminals at both ends of the capacitor at this time is detected by the voltage detection circuit connected by the fourth switching unit. The calculation section can estimate the power supply voltage from the detected voltage using an equation for calculating the power supply voltage including the capacity of the capacitor as a constant. The insulation resistance is calculated based on the estimated power supply voltage and the voltages detected by the detection unit after the second and third switching unit are unmade using an equation for calculating the insulation resistance including the capacity of the capacitor as a constant, which makes it possible to reduce an error in the measurement of the insulation resistance and to thereby improve the accuracy of detection of the state of insulation.
When a microcomputer is used in the voltage detection circuit of such an insulation detector, negative potential can be applied to an input port of the microcomputer for detecting the voltage across the capacitor as a result of opening and closing of the switching unit. For this reason a Schottky barrier diode is used as a diode for clamping the negative potential. However, since a voltage detected by the voltage detection circuit is used for the calculation of an insulation resistance in such an insulation detector, when the accuracy of the voltage detected by the voltage detection circuit is questionable, a problem arises in that accuracy cannot be improved in detecting the state of insulation. Under the circumstance, there are demands for an improvement of the detecting accuracy of the state of insulation through an improvement in the accuracy of a voltage detected by a voltage detection circuit.